1. Field of the Invention
The present invention relates to an irradiation dose calculation unit and an irradiation dose calculation method for calculating from prescription data irradiation doses from one or more portals (directions) to a target, and to a recording medium for recording a program for implementing the irradiation dose calculation method.
2. Description of Related Art
FIG. 18 is a view illustrating proton beam radiation therapy described in M. Urie, "Treatment Planning for Proton Beams", Ion Beams in Tumor Therapy published by CHAPMAN & HALL, pp. 279-289, 1995. In this figure, the reference numeral 1 designates a CT (Computed Tomography) of the head of a subject; 2 designates a target tumor for proton beam radiation; 3 designates a brain stem as a critical organ fragile to radiation; 4 designates a proton beam; 6 designates a bolus for varying the dose distribution in depth direction so as to focus a high dose portion on the location of the tumor 2; and 7a and 7b designate a collimator for matching the lateral beam width to the width of the tumor 2.
To apply the radiation therapy, the target is irradiated by the radiation beam while avoiding critical organs (the brain stem 3, in this case) as shown in FIG. 18. Although the proton bream radiation from a single portal is shown in FIG. 18, radiation from multiple portals (directions) is carried out in actual therapy, and doses from the portals are determined such that normal tissues and critical organs are protected from injury. Since the tolerable doses are known for normal tissues, the doses from the portals are empirically determined such that the doses absorbed by the normal tissues located in radiation paths do not exceed the tolerable doses.
Thus, the conventional irradiation dose calculation method empirically determines the doses from the portals such that the doses absorbed by the normal tissues located in the radiation paths become less than the tolerable doses. This method, however, presents a problem in that optimum doses to the target and critical organs are not always assured.